Some conventional systems and methods for accomplishing these goals rely upon scan mirrors that receive signals from an object and relay them into an aperture of an optical system—and, for response, conversely receive signals from the optical-system aperture and return those signals toward the object. Some such systems and methods instead (or also) rely upon large, mechanical gimbals that support and reorient the entire optical system. Both approaches entail relatively high moments of inertia, and accordingly large motors and elevated power requirements.
Such configurations require extremely adverse tradeoffs and compromises between, on one hand, undesirably high cost and size, and on the other hand structural weaknesses that lead to unreliability and even failure. For instance expensive custom parts and instrumentation are the rule rather than the exception, while some conventional devices have dimensions on the order of one to ten centimeters with mass of one to ten or even hundreds of kilograms.
These are significant handicaps for—in particular—devices that may be for use in airplanes and satellites. Even in these cases, such drawbacks might be acceptable if such systems provided superb performance, but unfortunately angular resolution in conventional systems of various types is generally no better than two-thirds of a degree—sometimes as coarse as ten degrees and more.
For example conventional macroscopic gimbal controls are most typically good to roughly one degree or less, although some units capable of precision in tens of microradians are available for millions of dollars each. Sensors using focal-plane arrays, e.g. quad cells, are typically precise to roughly ten degrees. Other nonmechanical systems include quad cells behind fisheye lenses.
The poor angular resolution and other performance limitations of such sensors arise in part from use of fixed, very large sensor assemblies, typically quad cells, CCD or CMOS arrays, at a focal plane—with fixed fields of view. These components accordingly also suffer from limited fields of regard. Furthermore the necessity for downloading into a computer memory the massive volumes of data from multimegabyte sensor arrays makes the frame rate of these systems extremely slow.
In efforts to improve the field of regard, the large areal arrays are sometimes placed behind radically wide-angle lenses, even fish-eye lenses. This strategy, however, is counterproductive in that it only compounds the data-download problem, while also yielding intrinsically coarse angular resolution and very nonlinear angular mapping.
In other words these systems are squeezed between the need for high resolution and the need for broad field of regard; this squeeze comes down to an all-but-prohibitive demand for dynamic range, or bandwidth. Data congestion, furthermore, is doubly problematic because in these systems the entire contents of every frame must be retrieved before that frame can be searched for an optical source of interest.
One rather unnoticed contributor to inadequate dynamic range is the direct relationship between gimbal angle or scan-mirror angle and excursion of the beam in the external scanned volume. That relationship is a natural one-to-one for a conventional gimbal system, and one-to-two for a rotating mirror. Since the direct effect of mechanical rotation is relatively slow for conventional macroscopic gimbals, and relatively limited in overall angular excursion for scan mirrors, the external beam-angle excursion is either slow or limited, or both.
In attempts to mitigate low resolution and frame rate, some workers have proposed to substitute a so-called “position-sensing detector” (PSD) for the commonly used larger arrays. The advantage of a PSD—which is a unitary device, not an array—is that it inherently locates and reports position of only a detected optical source, not an entire scene, and thus requires download of only a far smaller amount of data.
Another inherent advantage of a PSD is that it provides a continuous, analog positional readout, intrinsically yielding extremely high resolution. The report from an array is instead quantized by the pixel (or “aliasing”) effect that is central to any kind of array detection.
The PSD reports position on its own sensitive surface, in units of distance from its nominal center along two orthogonal axes. To find angular mapping, typically these off-center coordinates are divided by the focal length of a final focusing element.
Unfortunately these reported distances and therefore the angular mapping of a PSD are nonlinear, to the extent of several percent at the PSD edges—aggravating the analogous handicap introduced by a fish-eye or other wide-angle lens—and are also temperature sensitive. The detector may report accurately that an optical source has been sensed, but fail to report accurately where that object is, unless it is near the nominal center, or origin of coordinates.
It might be supposed—although in actuality this supposition is well beyond the present state of the art, and artisans of ordinary skill—that such a system could be quickly turned to look directly at the candidate object, for a more-accurate assessment of position. In any conventional detector, however, this solution is impractical due to the lumbering response of an associated macroscopic gimbal system, or even of a scan mirror that is redirecting the light into the detector aperture.
Often it is desirable to know something more about an optical source that has been noticed—the character of the light itself, and any intelligence signal that may be impressed upon that light. Accurate determination of wavelength and frequency modulation information, as may be gleaned from the foregoing discussion, is beyond the capabilities of these systems. Similarly infeasible is any exploration of physical objects that may be associated with the optical source.
The intractability of attempting to operate with such systems may be clarified by consideration of some practical situations which call for use of optical sensors. In most applications a person or an apparatus points a light source toward, most typically, some sort of vehicle—to guide an object in an attempt to rendezvous with the vehicle. Commonly the intention is adversarial, as for example damage to the vehicle; while the optical-sensor apparatus is mounted on the vehicle and its purpose is to detect the presence of the light beam and initiate some protective response.
Such response, usually intended to produce confusion as to the exact location of the vehicle, sometimes takes the form of returning a literally blinding flash of light toward the person or apparatus that is pointing the original source, to temporarily dazzle and confuse that source-controlling entity. Alternatively a response can be to eject from the vehicle many particles that strongly reflect the guide light, to instead confuse directional-control mechanisms of the moving object. Accompanying either of these may be an entirely different kind of response, namely an effort to disable the source-pointing person or apparatus, or the object. Such a disabling response, directed toward the object or source, may take the form of either a physical article or of powerful radiation. Still another desirable kind of response would be investigatory, i.e. determining the character of the guide beam or of the guided object; such information can be used to determine and report the nature of the guiding system itself, either for purposes of immediate efforts to confuse and avoid or for future protective-design work.
The person or apparatus pointing the source may be adjacent to the initial position of the object. In a sense this is the easiest case from the standpoint of protective response, because the source can be treated as a beacon for guidance of a disabling response that eliminates both the light source and the object—if the response is sufficiently prompt, so that the source and object are still not only in-line but also relatively close together. In another sense, however, this is a difficult case from the standpoint of confusion, because the object may have been designed to look (for its guidance) backward at the source rather than forward at the vehicle—in which event the ejection of reflecting particles cannot confuse the directional-control mechanisms of the object, as long as the pointing entity can keep the vehicle in view.
The person or apparatus pointing the source may, however, instead be at a different position—off to the side from the path of the object, and from a line between the source and the object. In this event, disabling both the source and object with a single response is not possible; but at least confusion can be more-readily produced since the object is necessarily designed to look forward at the vehicle, so that either the dazzling or the decoy-particle strategy, or both, can be effective.
One type of movable-mirror device that is known in various kinds of optical-detection systems is a single scan mirror of about 25 or 30 mm or more, consistent with the earlier statement of dimensions for conventional systems. Such mirrors are too bulky and heavy to overcome the above-discussed problems of response speed.
Another type of known movable-mirror device is a spinning cylinder with multiple mirrors carried on its outer surface. Such a polyhedral construction does provide a movable mirror, sometimes disposed along an optical path between a detector and an entrance aperture. Dimensions of each of the mirrors in such a device are typically in the tens of millimeters, also consistent with the previous indication of representative dimensions for conventional systems. Hence the overall device and even the individual mirrors are too big and heavy to free the optical-detection art from the response-speed and related limitations discussed above. These mirror wheels are ordinarily made to spin continuously; hence the individual mirrors of such an array lack independent maneuverability for customized control movements. Accordingly they are poorly suited for practical use in rapid detection and tracking of a particular source object.
Also of interest are telescopes—including astronomical telescopes—particularly of the type that has a movable mirror positioned between an entrance aperture and a detector. For present purposes, however, any interest in such devices is academic, as the movable components are huge and far too massive to be useful in any rapid-response system. Even more relevant is the typical limitation of field of view, in telescopes, to less than ten degrees.
Smaller deformable mirrors, too, are sometimes placed within optical systems in positions such as just described. A device of this type generally comprises a continuous reflective membrane that is controllably bent and distorted to correct wavefront errors. Such mirrors are typically at least 20 to 30 mm across.
Another type of known moving-mirror device is called a “microelectromechanical system” (MEMS) mirror. Such mirrors have never been associated with the field of optical-source detection that is under consideration here, heretofore. By “heretofore” I mean prior to the filing of my previous U.S. provisional '301 application mentioned above, and upon which this document is based.
Such devices, introduced some years ago by the Texas Instruments Company, and more recently in versions produced by Lucent Technologies and called an “optical switch”, most commonly take the form of arrays of very small mirrors—each on the order of ten to 500 microns across. At least in principle individual mirrors can be made available in the same format. In use these devices, while some are capable of continuous positional control, are most often only bistable, used for switching in optical information networks and also in an image-projection system for personal computers.
My above-mentioned provisional '301 application, and the intermediate PCT and U.S. national-stage application, introduce use of one or more such mirrors. In describing such mirrors these applications, and most particularly the U.S. national application, in most passages indicate that the mirror dimensions are limited to “a few millimeters”—or “merely by way of example, in a range from a few tens of microns wide to several millimeters or more”. It is also said that “the most preferable tested embodiments use e.g. silicon scan mirrors in the range of 1.5×2.1 mm”.
A New Generation of MEMS Mirrors
Those dimensional indications were based upon the specifications of suitable MEMS mirrors available at the time of that writing, which generally were piezodriven, electrostatically driven, or mechanically driven by piston mechanisms and the like. In the interim, patents and published applications have proposed much larger mirrors.
Although one such document suggests usability with lidar, it has not been suggested that these new devices be used in any configuration known in the above-defined “Field of the Invention” or, more particularly, set forth in my above-noted precursor patent applications—e.g. with the mirror or mirrors internal to the optical system or with field of view magnified. One physical system using such a large mirror in a configuration external to the optical system has been built, but such a configuration is (apart from the large mirror) substantially conventional.
The larger units are as fast and precise as the earlier ones, but at the same time capable of even larger angular excursions. The improvements appear mainly due to use of magnetic rather than the earlier piezoelectric, electrostatic or mechanical drives.
Magnetic forces generally extend farther from the drive components, thus allowing larger mirrors with greater clearances—as well as lower voltages, and less rigidity in the force coupling from drive to mirror. Mirrors having these new properties furthermore can be optimized in several different ways, to increase optical-energy throughput, while at the same time—as set forth in this document—actually enhancing the effective resolution in imaging.
Some such ways are set forth in the previously introduced patent publications of Draper and Corning—most particularly an early dual-axis configuration having a fixed magnetic field at forty-five degrees to the two rotation directions, and generally aligned within the rest plane of the rotatable mirror. The exact origin of that configuration is not specified, but it is shown and described as “prior art” in FIGS. 2 and 3 of both the '921 patent and the '234 published application.
The same drawings are also presented in this present patent document as FIGS. 7 and 8 (after Bernstein, Taylor et al.) respectively. The moving parts of that device apparently are MEMS components, formed in place by now-well-known procedures of etching and forming microscopic elements from an initially single silicon wafer or substrate, or by silicon-on-insulator “SOI” procedures, or the like.
That evidently seminal two-axis geometry has a generally square mirror BT33 (FIG. 7) formed on a square pad BT32 between two torsional hinges or so-called “flexures” BT40, which are positioned at two opposed sides of the square pad. The ends of the flexures BT40 that are remote from the square pad BT32 attach to the inside edge of another square structure that is only a narrow frame, i.e. a square annulus BT34, so that the square mirror BT33 rotates about a first axis defined through its two flexures BT40 within the square annular frame BT34.
The latter frame, in turn, is mounted between two like flexures BT41 but at orthogonal positions, i.e., along edges of the square frame BT34 that are orthogonal to the positions of the two flexures BT40, first mentioned in the preceding paragraph, and the ends of these flexures BT41 remote from the square annular frame are mounted to inside edges of an outer square body BT35. These latter two flexures enable the square mirror BT33, and the square frame BT34 around it, to rotate together about a second axis—orthogonal to the first—defined through the outer two flexures BT41.
Accordingly the square annular frame BT34, with its outer square body BT35, is ingeniously made to function as a microminiature gimbal box. It provides rotation about two orthogonal axes, very generally as did the classical, mechanical gimbal boxes—but some three or more orders of magnitude smaller and faster.
Also formed on the front of the mirror pad BT32 (sharing that surface with the mirror BT33) and on the gimbal frame BT34 are numerous fine conductive traces BT36, BT38 respectively, which are disposed and connected to function as conductive coils. Electrical currents passing through these coils, via the flexures BT40, BT41 respectively, establish variable, controllable magnetic fields mainly oriented in and out of the plane of the drawing (and of the pad and frame)—that interact with the fixed field lines B to forcibly drive the mirror pad BT32 and gimbal frame BT34 in their two respective orthogonal rotations.
Although this two-axis MEMS mirror configuration appears to be a fully functioning single mirror, the Draper and Corning patent documents assert major improvements in magnetically driven MEMS mirrors. In particular the '901 published application preserves the same basic square geometry of mirror, pad and gimbal frame—with torsional flexures positioned (in almost all its embodiments) along the sides of the square.
One of its principal areas of improvement is in making a much greater fraction of the pad space available for reflection of optical energy by the mirror, simply by forming the coils on the back face of the pad rather than on the reflective mirror face. This change enhances the intrinsic size benefit of magnetically driven MEMS mirrors. The '901 document further exhibits such coils of many different configurations—spirals, loops etc.—for a variety of purposes.
A second main area of advancement in the '901 application is in orientation of the stationary magnetic field perpendicular to the rest plane of the mirror, rather than in that plane. Yet a third refinement is in ganging large numbers of the magnetically driven mirrors together, to form an array capable of complex switching tasks—or, alternatively, capable of operation with plural or multiple mirrors working in common to steer optical beams of large cross-section.
One of the many drawings in the '901 application shows flexures in the corners, rather than along the sides, of the square mirror pad and square gimbal frame. That application, however, does not make much of a point of this divergent geometry.
Thus it is left for the '921 patent to elaborate such a point, namely that corner placement of the flexures leaves a much greater fraction of the mirror-pad area available for oval or round mirrors. These mirrors are accordingly able to reflect much larger laser (or other) light beams of oval or round cross-section, for the same size mirror pad.
(More generally the '921 patent explores multimirror options and coil-configuration variations. By contrast the '901 application focuses mainly upon the basic superstructure of torsional mirror and gimbal-frame suspension etc.)
Many additional important optimizations are featured in the '921 patent, including stacks of coil layers, on both faces of the mirror pads rather than only the reflective face. Such geometry yields not only larger currents and larger magnetic fields—and therefore greater torque and more-nimble acceleration—but also far more-complex control capabilities. Some of the coils are counterwound relative to one another.
Some coils are disposed on just e.g. half of the movable pad area, paired on opposite sides of the rotational axis—as, for example, in FIGS. 8A, 8B of the '921 patent, reproduced in this document as FIGS. 9 and 10 (after Bernstein, Taylor et al.) respectively. As will be seen in later sections of this document, such separable coils BT76, BT78 (FIG. 9) and BT84, BT86 (FIG. 10) may offer special benefits for present purposes. Of course no such suggestion appears in the '921 patent under discussion.
More generally, the several above-outlined variations in coil geometry better exploit the directionality of the magnetic energy, actively twisting the pad about its flexures to still further increase the agility of the pointing function. These several new features are also combined to achieve a highly efficient use of the overall space and distances within an array of these mirrors—or in other words a high so-called “fill factor”.
Fill factor is sometimes defined as a ratio of aggregate area of the reflectors to overall area of the array. An alternative figure of merit is a “linear” fill factor, defined as aggregate linear dimension of the reflectors (along one or another direction of the array) to corresponding overall linear dimension of the array.
This '921 patent also adds a system for detecting and measuring mirror position—using the same coils that drive rotation—by introducing onto the coils a high-frequency carrier that is then modulated by angular offset of the mirror- and gimbal-frame-mounted coils. Electrical filters separate the high-impedance positional modulation, carried at the high frequency, from the high-power drive signals.
Furthermore the '921 patent teaches that the permanent magnets and the coils can be reversed in position—i.e. placing the magnets on the backs of mirror pads, and the coils on the stationary base. The patent also describes noteworthy benefits of such new configurations.
Yet another group of significant refinements is introduced by the published '234 patent application. These include a different approach to raising the mirror fill factor: here each mirror pad carries a “lid” that overhangs the gimbal frame—and if desired also overhangs the surrounding stationary outer square body mentioned above.
Each mirror is formed and sized to match the oversize lid, rather than only the internal mirror pad. As a result the mirror dimensions are not at all limited by those of the pad or even the gimbal frame. The '234 application asserts fill factors as high as “about 80% or higher (e.g., about 95%”)—which represent phenomenal improvements.
This overbite approach to fill-factor enhancement appears to be an elegant solution. These advances, as well as the fill-factor improvements of the '921 patent noted earlier, naturally are importance only for applications suited to mirror arrays, as distinguished from individual mirrors.
The '234 application also further develops the theme of permanent-magnet placement on the moving-mirror pad, rather than associated with the underlying base. Alternatively in its FIG. 7 the '234 patent still further teaches, perhaps somewhat radically, mounting the variable-current coils and a stationary permanent magnet together—in the base, below the moving mirror.
There the stationary coils generate magnetic-field contributions that augment or partially oppose the fields of the intimately adjacent permanent magnet. In this scheme only a thin piece of magnetically passive soft metal is mounted to the moving mirror, providing enough magnetic reluctance to interact with the permanent and variable magnets in the base and thereby activate rotation of the mirror. Neither a permanent nor an active variable magnet need ride on the mirror.
The remaining Taylor-Bernstein patents principally address details of making and positioning coils, and flexure geometries to optimize linear fill factor, in multimirror arrays. They may have application in those of my inventions that are amenable to use of multimirror arrays.
Again, use of these larger mirrors in the environment of my present and related inventions is not suggested in the known art.
Afocal Lenses
Another familiar optical device not heretofore associated with the present field, are afocal lens packages used e.g. as lens focal-length extenders. These are commonplace in ordinary cameras.
Almost all the optical devices discussed above, and most conspicuously the astronomical ones and MEMS devices, are known only in different fields from the present invention.
As can now be seen, the related art fails to resolve the previously described problems of the above-defined “Field of the Invention”—most particularly slow tracking, small steering-mirror dimensions leading to low signal-to-noise ratio and therefore limited pointing precision and accuracy, and overly constrained field of view. The efforts outlined above, although praiseworthy, leave room for considerable refinement.